In recent years, machining techniques for increasing density and micronization are becoming ever more important in manufacturing steps for semiconductor elements. One such technique, chemical mechanical polishing (CMP), has become an essential technique in steps for semiconductor elements, for formation of Shallow Trench Isolation (STD, flattening of premetal insulating films and interlayer insulating films, and formation of plugs and embedded metal wirings.
In most CMP steps, polishing proceeds by pressing a film to be polished of a substrate on which the film to be polished has been formed against a polishing pad, and relatively moving the substrate and the polishing pad while supplying a polishing agent between the film to be polished and the polishing pad. The polishing agent and polishing pad is the major factor that determines the polishing properties including the polishing rate and flatness of the film to be polished, the polishing selectivity, the number of scratches and the in-plane uniformity of the substrate. In particular, the polishing properties vary significantly depending on the type of abrasive and additives in the polishing agent and the material and hardness of the polishing pad.
The polishing agents commonly used for CMP steps are polishing agents containing silicon oxide (silica) abrasives such as fumed silica or colloidal silica. It is known that a wide range of different films can be polished by appropriately selecting the abrasive concentration, pH and additives in the silica-based polishing agent.
Demand is also increasing for polishing agents comprising cerium compound abrasives, mainly designed for silicon oxide-based insulating films such as silicon oxide films. For example, ceria-based polishing agents comprising cerium oxide (ceria) particles as the abrasive are known to allow polishing of silicon oxide-based insulating films at high rate, with lower abrasive concentrations than silica-based polishing agents.
It is also known that addition of appropriate additives to ceria-based polishing agents can improve flatness and polishing selectivity.
In STI-forming steps, for example, a silicon oxide-based insulating film, as the film to be polished, is formed on a silicon substrate to embed the silicon oxide-based insulating film in trenches created at the silicon substrate. In such steps, irregularities reflecting the step heights of the trenches created in the lower silicon substrate are generally produced on the surface of the silicon oxide-based insulating film. Appropriate selection of the additives added to the ceria-based polishing agent for polishing of the silicon oxide-based insulating film can increase the polishing rate ratio on the convexities with respect to the polishing rate on the concavities of the silicon oxide-based insulating film. As a result, the convexities of the silicon oxide-based insulating film are polished preferentially compared to the concavities, and therefore, the post-polishing flatness can be improved.
Moreover, it is common in STI-forming steps to provide a silicon nitride film or polysilicon film (pSi film) as a polishing stop layer at the lower layer of the silicon oxide-based insulating film. Appropriate selection of the additives added to the ceria-based polishing agent for polishing of the silicon oxide-based insulating film can increase the polishing rate ratio (the polishing selective ratio) for the silicon oxide-based insulating film with respect to the polishing stop layer. As a result, it is easier to stop polishing when the polishing stop layer has been exposed, and the polishing stop property can be increased. Cerium oxide (ceria)-based polishing agents to be used in CMP steps are disclosed in Patent document 1 and Patent document 2, for example.
Incidentally, with the micronization of circuit dimensions of semiconductor devices in recent years, scratch generated during CMP steps is becoming a serious problem. This is because scratch produced in the film to be polished during the CMP steps causes wire breakage and shorts in microscopic transistors and interconnections.
Methods for reducing the number of scratches have been proposed, wherein the mean particle size of the abrasive in the polishing agent is reduced. For example, Patent document 3 describes a polishing agent wherein fine tetravalent metal hydroxide particles are used as the abrasive.
Patent Document 3 teaches that the polishing agent comprising tetravalent metal hydroxide particles reduces the number of scratches compared to conventional ceria-based polishing agents. Furthermore, Patent document 3 also teaches that a prescribed polishing selective ratio can be obtained with a polishing agent comprising the tetravalent metal hydroxide particles and prescribed additives.